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LECTURE22, 23, 24: CHLORALKALI INDUSTRYSODAASH (SODIUM CARBONATE, NA2CO3)
CAUSTIC SODA (SODIUM HYDROXIDE, NAOH)
CHLORINE (CL2)
CHEMICAL TECHNOLOGY (CH-206)
Department of Chemical Engineering
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SODIUM CARBONATE (NA2CO3)
Sodium carbonate (Na2CO3)also known as
washing soda or soda ash, is a sodium salt of
carbonic acid.
Most commonly occurs as acrystallineheptahydrate, which readily effloresces to form a
white powder, the monohydrate.
Sodium carbonateis domestically well known as
awater softener.
Soda ash is the most important high tonnage, low
cost, reasonably pure, soluble alkali available to
the industries as well to the laboratory.
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SODIUM CARBONATE:MANUFACTURING PROCESSES
1. Leblanc process.
2. Solvays ammonia soda process.
3. Dual process (modified Solvays process)
4. Electrolytic process.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC PROCESS
Raw materialsBasis: 1 ton Sodium carbonate (98% yield)
Common salt 1126kg (sea water, salt lake and
sub soil water )
Sulfuric acid 945kg (contact process )
Lime stone 963kg (mineral calcite or aragonite)
Coke 463kg
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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC
PROCESS
Reactions
NaCl + H2SO4NaHSO4+ HCl (a)
NaHSO4+ NaClNa2SO4+ HCl (b)
Na2SO4+4CNa2S + 4CO (c)
Na2S +CaCO3Na2CO3+ CaS (d)
(Black ash sludge)
CaS + H2O + CO2
CaCO3+ H2S (e)CaS + H2SCa(HS)2 (f)
Ca(HS)2+ CO2+ H2OCaCO3+ 2H2S (g)
H2S + OH2O + S (h)
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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC PROCESS
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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC PROCESS
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(a) (b) (c,d)CaS is
separated
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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC PROCESS
NaCl is mixed with the conc. H2SO4in equivalent quantities and heated
in a cast iron salt cake furnace by flue gases from adjacent coal of fire.
NaHSO4along with HCl gas is formed.
HCl is passed to tower packed with coke and is absorbed through a spray
of water comes down in the tower.
The paste of NaHSO4is taken out and heated to a high temperature on
the hearth of a furnace along with some more common salt.
NaHSO4is thus converted into sodium sulfate, known as salt cake.
The salt cake is broken or pulverized, mixed with coke and limestone
and charged into black ash rotary furnace consisting of refractory lined
steel shells.The mass is heated by hot combustion gases entering at one end and
leaving at the others.
Themolten porous gray massthus formed known asblack ashis
separated from the calcium sludge and then crushed and leached with
water in absence of air in a series of iron tank.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC PROCESS
The extract containing Na2CO3, NaOH, and other
impurities is sprayed from the top of a tower in
counter current to flow of hot gases from the black ash
furnace.
The sodium carbonate thus obtained is concentratedin open pans and then cooled to get sodium carbonate.
The product is calcined to get soda ash which is re-
crystallized to Na2CO3.10H2O.
The sludge containing mostly CaS is left behind asalkali waste.
The liquor remaining after removal of first batch of
soda ash crystals is purified and then causticized with
lime to produce caustic soda.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:LEBLANC PROCESS
Recovery of sulfur from alkali wasteAlkali waste is charged into cylindrical iron vessels
arranged in series and CO2delivered from lime kilns is
passed through it, the H2S gas thus obtained is then
conduced together with a regulated amount of air in aClaus kiln containing iron oxide as catalyst.
The exothermic reaction proceeds without further
external heat.
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CaS + H2O + CO2CaCO3+ H2S (e)
CaS + H2SCa(HS)2 (f)
Ca(HS)2+ CO2+ H2OCaCO3+ 2H2S (g)
H2S + OH2O + S (h)
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
PROCESS
Raw materialsBasis: 1 ton sodium carbonate
Salt 1550kg (sea water, salt lake
and sub soil water)
Limestone 1200kg (mineral calcite oraragonite)
Coke 90kg
Ammonia as a catalyst 1.5kg (Loss)
High pressure steam 1350kgLow pressure steam 1600kg
Cooling water 4000060000kg
Electric power 210 KWH
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
PROCESS
Reactions
CaCO3CaO + CO2 H = +43.4kcals
C(s) + O2(g)CO2(g) H = 96.5kcals
CaO(s) + H2O (l)Ca(OH)2(aq) H = 15.9kcals NH3(aq) +
H2O(l)NH4OH(aq) H = 8.4kcals
2NH4OH + CO2(NH4)2CO3+ H2O H = 22.1kcals
(NH4)2CO3+ CO2+ H2O2NH4HCO3
NH4HCO3+ NaClNH4Cl + NaHCO3
2NaHCO3Na2CO3+ CO2+ H2O H = +30.7kcals
2NH4Cl + Ca(OH)22NH3+ CaCl2+ 2H2H = +10.7kcals
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CaCO3
+ 2NaClNa2
CO3
+ CaCl2
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PROCESS
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
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Preparation and purification of brineSaturated solution of NaCl is used.
Brine contains impurities such as calcium, magnesium and iron
compounds.
To remove calcium sulfate, magnesium and iron salts sodium
carbonate and sodium hydroxide are added.The precipitated carbonates and hydroxide are removed by filtration.
Sometimes sulfate are removed with BaCl2or the hot brine is treated
with OH and CO32ions.
The calcium, magnesium and iron salts from saturated brine may be
precipitated by dilute ammonia and CO2in a series of washingtowers.
The brine is purified by allowing it to settle in vats, as a result of
which precipitated CaCO3, MgCO3, Mg(OH)2and iron hydroxide
settle down and pure brine solution is pumped to the ammonia
absorber tower, where it dissolve NH3with the liberation of heat.
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Ammoniation of brineThe purified brine is allowed to percolate down the ammonia tower in
which ammonia gas is passed through the bottom in a counter current
fashion.
The brine solution thus takes up the necessary amount of ammonia and
liberates heat.
The gas which escapes solution in the tank is absorbed by the brine falling
down the tower.
Some carbon dioxide is also absorbed by ammonia, as a result of which
some insoluble carbonate is also precipitated.
The ammoniated brine is allowed to settle, cooled to about 30C and
pumped to the carbonating tower.
Carbon dioxide formationLimestone is calcined to get CO2in a lime kiln filled with coke.
As a result of burning of coke necessary heat required for the
decomposition of lime stone is generated.
CaO obtained from the lime kiln is converted into slaked lime and pumped
to the ammonia recovery tower.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
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Carbonation of ammonium brineCO2from the lime kiln is compressed and passed through the
bottom of carbonating tower down which ammoniated brine
percolates.
Carbonating towers operated in series with several precipitation
towers are constructed of cast iron having 2225 m height, 1.6
2.5 m in diameter.
During the precipitation cycle, the temperature is maintained
about 20-25C at the both ends and 45-55C at the middle by
making use of cooling coils.
The tower gradually becomes flooded as sodium bicarbonatecakes on the cooling coils and shelves.
The cooling coils of the foulded tower are shut off.
Then the fresh hot ammoniated brine is fed down the tower in
which NaHCO3are dissolved to form ammonium carbonate
solution.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
PROCESS
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Carbonation of ammonium brineThe solution containing (NH4)2CO3, unconverted NaHCO3
is allowed to fall down a second tower, called making
tower.
The making, towers are constructed with a series of boxesand sloped baffles.
Ammoniated brine and CO2gas (90-95%) from the
bicarbonate calciner is recompressed and pumped to the
bottom of the making tower.
The ammonium carbonate first reacts with CO2to form
ammonium bicarbonate and the latter reacting with salt,
forms sodium bicarbonate.
The heat of exothermic reaction is removed by cooling
coils.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
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FiltrationNaHCO3slurry is then filtered on a rotary vacuum
filter which helps in drying of bicarbonate and in
recovering ammonia.
The filter cake after removal of salt and NH4Cl bywashing with water, sent to a centrifugal filter to
remove the moisture or calcined directly.
During washing, about 10% NaHCO3also passes into
filtrate.The filtrate containing NaCl, NH4Cl, NaHCO3and
NH4HCO3is treated with lime obtained from lime
kiln to recover NH3and CO2.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
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CalcinationNaHCO3from the drum filter is calcined at about 200C in
a horizontal calciner, which is either fired at feed end by gas
or steam heated unit.
The heating being through the shell parallel to the product,
which prevent the formation of bicarbonate lumps.
The hot soda ash form the calciner is passed through a
rotary cooler and packed in bags.
The exit gases (CO2, NH3, steam etc.) are cooled and
condensed to get liquid ammonia; the rich CO2gas is cooled
and returned to the carbonating tower.
The product from the calciner is light soda ash.
To produce dense soda ash, sufficient water is milled with it
to form more mono hydrate Na2CO3.H2O and the mixture is
recycled.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
PROCESS
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Recovery of ammoniaThe NH3is recovered in strong NH3liquor still, consisting of two sections.
The parts above and below the lime inlet is called as heater and lime still,
respectively.
The filtrate obtained from washing of NaHCO3from the pressure type rotary
filter is fed into the heater, where free NH3and CO2are driven off by distillation.Dry lime or milk of lime (slaked lime) obtained from lime kiln is fed through the
lime inlet and mixed with the liquor from the heater.
As the liquor flows down the column, calcium chloride and calcium sulfate are
formed and NH3gas is released.
NH4Cl + Ca(OH)2CaCl2+ 2NH3+ H2O
(NH4)2SO4+ Ca(OH)2CaSO4+ 2NH3+ 2H2OThe liquor from the bottom of the lime still is free from ammonia and contains
unreacted NaCl and largely CaCI2, which is disposed off.
The liquor is, therefore allowed to settle in settling ponds and the clear liquid is
evaporated till the salt separates out and is sold as such for calcium chloride or
further evaporated.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:SOLVAY'S AMMONIA SODA
PROCESS
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Kinetics and thermodynamicsThe overall reaction shows that salt and calcium
carbonate are the only raw materials which are
continuously supplied in the process and that produce
sodium carbonate and calcium chloride
CaCO3+ 2NaClNa2CO3+ CaCl2Overall reaction of ammoniation of brine and then
treatment of carbon dioxide to ammoniated brine is as
under
2NaCl + 2H2O +2NH3+ 2CO22NaHCO3+ 2NH4ClThe above reaction shows the role of ammonia and carbon
dioxide in the process, and also determines the yield of
the final product.
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Kinetics and thermodynamicsThere so conditions favourable to it are precisely
defined. For these the reaction is divided in to two
steps.
2NH3+ 2CO2+2H2O2NH4HCO3---- (a)2NaCl + 2NH4HCO32NaHCO3+ 2NH4Cl ---- (b)
Reaction (a) is favoured by low temperature because it
requires the dissolution of gas in water, is displaced to
right by virtue of the fact that reaction (b), which
utilizes the product by subtracting it from (a) isdisplaced in the same direction.
Consequently, it is the precipitation of NaHCO3
according to (b) which is the driving force behind the
entire method.
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Above data indicate that precipitation fortunately tends to take place preferentially
with satisfactory yields.
On the basis of data and common ion effect on precipitation of salts, physicochemicalconditions most suitable for the forward step of reaction (b) which causes
precipitation of NaHCO3are as under:To maintain lowest possible temperature in order to lower the solubility off sodium bicarbonate
To maintain the greatest possible concentration of one or both the salts appearing on the
product side of reaction (b) with the aim of lowering still further solubility of sodium
bicarbonate.
The solubilities of the salts at various temperature is as under
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Kinetics and thermodynamicsThese conditions are nevertheless discerningly applied because they
serve to bring about appreciable increase in the yields of NaHCO3
and permit the most effective use of most costly reagent NH4HCO3
in reaction (b).
Attention is paid to the fact that, if precipitation temperature isalways kept low, the sodium bicarbonate separates in a
microcrystalline form which is with difficult to filter and it is soluble
during subsequent washing on the filter, increase requirement of
NaCl.
Experimentally, the conditions which are most effectively reconcile
the physicochemical aspect of precipitation of sodium bicarbonateeconomically are as under284gm/liter (4.9mole/liter) of NaCl reacting with 76gm/liter (4.5mole/liter)
of NH3instead of equimolecular solution of two reagents.
Relatively high temperature (60-650C) at the start so as to allow the
formation of well-developed NaHCO3crystallization seeds and increasing the
volume of these seeds to decrease the solubility of salt with gradual cooling.
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Major engineering problemAbsorption units
Making tower
Development of suitable calcining equipment
Filtration unit
Ammonia recovery
Waste disposal
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SODIUM CARBONATE:MANUFACTURING PROCESSES:DUAL PROCESS
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Raw materialsBasis: 1 t Sodium carbonate
Crystalline Salt 1260kg
Ammonia 325kg (HaberBosch Process)
High pressure steam 1350kg
Low pressure steam 100kg
Cooling water 5000080000kg
Electric power 450KWH
Co-product (NH4Cl) 620kg
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SODIUM CARBONATE:MANUFACTURING PROCESSES:DUAL PROCESS
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Reactions
C + O2CO2
NH3+ H2ONH4+ OH
CO2+ H2OHCO3+ H+
CO2+ OHHCO3
Na++ Cl+ NH4++ HCO3
NH4+Cl+ NaHCO3
2NaHCO3
Na2CO3+ CO2+ H2O
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SODIUM CARBONATE:MANUFACTURING PROCESSES:DUAL PROCESS
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The liquor from carbonation tower, containing ammonium chloride,
unreacted NaCl and traces of sodium carbonate is ammoniated in
ammonia absorber.
The ammoniated liquor is sent to a bed of washed salt in salt dissolver.
The resulting liquor is gradually cooled to 00C in refrigerating tank unit,
resulting into crystallize out ammonium chloride.The slurry containing ammonium chloride is thickened and NH4Cl is
centrifuged and dried, which is a co-product.
These is the principal modification of dual process in which ammonium
chloride is recovered as co-product rather than liberation of the
contained ammonia for recycle as in the Solvay process.
The liquor obtained after separation of NH4Cl is charged to series of
carbonation towers in which CO2is passed from bottom in the counter
current flow of liquor.
The resulting sodium bicarbonate is thickened into thickener and
centrifuged. It is then calcined into sodium carbonate.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:DUAL PROCESS
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Major engineering problemSalt purification
Corrosion
Refrigeration Cost
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SODIUM CARBONATE:MANUFACTURING PROCESSES:DUAL PROCESS
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Advantage of Solvay
processLess electric power
Less corrosion
problemUse of low grade brine
Not a problem of
disposal of co-product
Does not requireammonia plant
Disadvantage of Solvay process
Higher salt consumption
Waste disposal of CaCl2
brine stream
Higher investment in NH3
recovery units than
crystallization unit of NH4Cl
More steam consumption
Higher capacity plant set up
require for economic breakeven operation (100 v/s
55tons/day)
NH4Cl can be used as mixed
fertilizer ingredient which
minimizes the disposal
problem of Duel process.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:ELECTROLYTIC PROCESS
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Raw materialsBasis: 1 t Sodium carbonate (98% yield)
Salt 563kg (sea water, salt lake and sub soil water)
Carbon dioxide 424kg
Reactions
NaClNa++ Cl 2H2O + 2eH2+ OHAt cathode
2H2O + 2eH++ 2OH
Na++ OH NaOH
2NaOH + CO2Na2CO3+ H2OAt Anode
CleCl
Cl + Cl
Cl2
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SODIUM CARBONATE:MANUFACTURING PROCESSES:ELECTROLYTIC PROCESS
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At cathode (Steel tube)2H2O + 2eH++ 2OH
Na++ OH NaOH
2NaOH + CO2Na2CO3+ H2O
At Anode (Graphite)
CleCl
Cl + ClCl2
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SODIUM CARBONATE:MANUFACTURING PROCESSES:ELECTROLYTIC PROCESS
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Electrolytic cell consists of a perforated steel tube having a thin
lining of asbestos on the inside.
Brine is placed inside the cathode tube and a graphite rod is
immerged in it acts as anode.
When an electric current is passed, the salt solution undergoes
electrolysis and its ions pass through the diaphragm as a result of
electrical migration.
Hydrogen and caustic soda are formed at the cathode and chlorine
at the anode.
Hydrogen gas is allowed to escape through an opening provided at
the top of the cell.
Chlorine liberated at the anode is led away through a pipe and
compressed into steel cylinders.
The space between the cathode and outer tank is kept full of
steam and Carbon dioxide.
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SODIUM CARBONATE:MANUFACTURING PROCESSES:ELECTROLYTIC PROCESS
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Sodium ions pass through the asbestos and reach
the cathode, where H+ions and OH Ions are formed
as a result of reduction of water.
Hydrogen escapes through an opening at the top and
Na+ions combine with OH ions to form caustic soda.
Sodium hydroxide is reacted with pressurized CO2
yielding Sodium carbonate which is collected from
bottom of the cell.
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SODIUM CARBONATE:PROPERTIES
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Molecular formula : Na2CO3
Molecular weight : 105.978gm/mole
Appearance : White crystalline solid
Odour : Odourless
Boiling point : 16330CMelting point : 8510C
Density : 2.54gm/mL (Anhydrous)
Solubility : Soluble in water
99%sodium carbonate (58%Na2CO3) is known as light sodaash (solid density 1.86).
Dense soda ash has solid density of 1.91. Both grads (lightly
and dense) are granular.
Na2CO3.10H2O is known as washing soda.
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SODIUM CARBONATE:APPLICATIONS
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Widely used in the manufacture of glass,
Used in manufacture of sodium bicarbonate, caustic
soda,
Used in soap, pulp and paper, textiles industries
Used in petroleum and dyes industries
Used in foods, leather and water softening
industries.
As a photographic film developing agentAs an electrolyte
As a washing soda in household uses.
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SODIUM HYDROXIDE
(NAOH)
CHLORINE (CL)
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SODIUM HYDROXIDE (NAOH)
Sodium hydroxide (NaOH), also known as lye and
caustic sodais a highly caustic metallic base which is
awhite solid available in pellets, flakes, granules,
and as 50% saturated solution.
Caustic soda is produced as co-products by theelectrolysis of brine.
In India 80% caustic soda and more than 95% chlorine
are produced by electrolysis of brine.
Various commercial cells have been developed in order
to keep the anode and cathode products separate fromone another.Diaphragm cell
Mercury cathode cell
Membrane cell
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SODIUM HYDROXIDE (NAOH): TYPES OF
CELLS
Diaphragm cellSubmerged Cells
Cathodes remain submerged in this type of cell.
Graphite is used as anode.
The liquid in the cathode compartment is at low-level inorder to prevent the back flow of OH ions by diffusion, e. g.
Hooker and Townsend cells.
Dry Diaphragm CellsThe diaphragm cells contain a porous asbestos diaphragm
which permits a flow of brine from the anode to cathode and
prevents the mixing of anode product and cathode products.Graphite is used as an anode.Electrolysis starts with dry or empty cathode compartment,
e.g. Nelson, Gibbs and Vorce cells.
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SODIUMHYDROXIDE:DIAPHRAGM
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SODIUM HYDROXIDE: DIAPHRAGM
CELLSUBMERGED CELL: HOOKER CELLS
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SODIUMHYDROXIDE:DIAPHRAGM
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SODIUM HYDROXIDE: DIAPHRAGM
CELLSUBMERGED CELL: HOOKER CELLS
Hooker cells are cubic in shape with capacity from 10000 amp to 55000
amp.
It has concrete cover at the base from which flat blades of the graphite
projects upward and act as anodes which is supported vertically by a layer
of lead cast concrete base.
90 graphite anodes, each has dimension of 46163 cm are used.
The cathode consisting of flat steel fingers are supported horizontally from
the side steel frame extending inwards, from two sides so as to fit between
the rows of anode blades.
Concrete cover has inlet for brine and exit pipe for chlorine gas.
This concrete cover also projects the cast lead forming the condenser to the
anodes from attack by cell liquor.The cathode assembly has hydrogen and caustic off takes and the cathode
connection.
The cathode is directly covered with asbestos and forms the diaphragm,
which is completely submerged.Diaphragm is applied by dipping the cathode into a bath of asbestos slurry and the
asbestos is drawn into the screen by applying a vacuum to the hydrogen outlet.
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SODIUMHYDROXIDE:DIAPHRAGM
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SODIUM HYDROXIDE: DIAPHRAGM
CELLSUBMERGED CELL: HOOKER CELLS
WorkingA feed of brine between anode and cathode
compartment maintains the separation of anode
products from cathode products.
The brine passed into the anode compartment of thecell through the concrete cover and liberated chlorine
at the anode escapes through the cell cover.
Hydrogen liberates at the steel cathodes and the
weak brine containing caustic soda is withdrawn
through the hollow rectangular channel frames at theside.
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SODIUM HYDROXIDE: DIAPHRAGM CELL
DRY/POROUS DIAPHRAGM CELLS: NELSON CELLS
Reactions
NaClNa++ Cl H = + 97.2kcals
2H2O + 2eH2+ 2OH H = + 68.3kcals
At cathode
2H2O + 2e2H++ 2OH
Na++ OH NaOH H = + 112.0kcals
At Anode
CleCl
Cl + ClCl2
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SODIUM HYDROXIDE (NAOH)
DRY/POROUS DIAPHRAGM CELLS: NELSON CELLS
Nelson cell consists of a perforated steel tube having a thin
lining of asbestos on the inside.
The steel tube acts as the cathode and is suspended in an
outer steel tank.
Brine is placed inside the cathode tube and a graphite rod,
which acts as anode, is immerged in it.
The brine undergoes electrolysis by passing current and ions
of salt are passing through the diaphragm due to electrical
migration.
Sodium ions pass through the asbestos and reach the
cathode, where H+ions and OH ions are formed as a result
of reduction of water.
Hydrogen escapes through an opening at the top and Na+
ions combine with OH ions to form caustic soda, which is
collected at the bottom of the outer tank.
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SODIUM HYDROXIDE (NAOH)
DRY/POROUS DIAPHRAGM CELLS: NELSON CELLS
Hydrogen and caustic soda are formed at the
cathode and chlorine at the anode.
Hydrogen gas is escape through outlet provided at
the top of the cell, while caustic soda is collected at
bottom and withdrawn from time to time.Chlorine liberated at the anode is led away through
a pipe and compressed into steel cylinders.
The space between the cathode and outer tank is
kept full of steam, which acts in two ways.It heats the electrolyte and thus reduces its resistance
Keeps the pores of the asbestos diaphragm clear which
make migration of ions easy.
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SODIUM HYDROXIDE: MERCURY CATHODE CELLS
THE CASTNER KELLNER CELL
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It was introduced by Castner and Kellner in 1892
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SODIUM HYDROXIDE: MERCURY CATHODE CELLS
THE CASTNER KELLNER CELL
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Castner Kellner cell consists of large rectangular
tank with a layer of mercury at the bottom and
divided into three compartments by the state
partition which does not touch the bottom of cell.
Movement of eccentric wheel H comforts thecirculation of mercury from one compartment to
another.
Each of the side compartments called A, is fitted
with graphite anodes dipping in brine, whereas aseries of iron roads suspended in the middle
compartment act as cathodes.
The compartment contains a dilute solution of soda.
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SODIUM HYDROXIDE: MERCURY CATHODE CELLS
THE CASTNER KELLNER CELL
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When the electric current is passed, the
electrolysis of brine takes in the outer
compartment A.
Chlorine is liberated at the anode and is led away
through an exit provided at the top.Sodium ions are discharged at the mercury layer
which acts as cathode by induction.
It should be noted that H+ion will not be
discharged because of high over potential over themercury.
Na++ eNa (At cathode)
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SODIUM HYDROXIDE: MERCURY CATHODE CELLS
THE CASTNER KELLNER CELL
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The liberated sodium atoms dissolved in the mercury to from a
sodium amalgam which comes into the central compartment
due to the rocking motion given to the cell by eccentric wheel H.
In the compartment the Hg layer acts as an anode.
As a result of electrolysis of NaOH solution present in central
compartment, OH ions and Na+ions are formed.The OH ions move to the mercury anode and after getting
discharged react with the sodium atom presents in the
amalgam to form sodium hydroxide.
At the same time, the H+ ions furnished by slight dissociation
of water get discharged as hydrogen which escapes through exitabove the middle compartment.
The caustic soda solution is sufficiently concentrated(above
20%) it is removed periodically and concentrated to get fused
caustic soda.
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SODIUM HYDROXIDE: MEMBRANE CELLS
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A semi-permeable membrane is used to separate the anode and
cathode compartments. Membrane is porous chemically active plastic sheet that allow Na+ion
to pass but rejects the OH ions.
While in diaphragm cells, back migration of ion is controlled by the
rate of flow of fluids through the diaphragm and this is regulated by
careful control of liquid level in the compartments.
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SODIUM HYDROXIDE: MEMBRANE
CELLS8/
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Du Pont has developedper sulfonic acid polymer (Nafion)for
membrane while Ashai uses a multiple layer membrane ofper
fluorosulfonic acid polymer.
The purpose of membrane is to exclude OH and Cl ions from
the anode chamber, thus making the product far lower in salt
than that from diaphragm cell.
A membrane cell 20 times larger than diaphragm can produce
240 ton of chlorine per year and power consumption is
satisfactory reduced below either mercury or diaphragm cells.
A bipolar cell unit is capable of producing 20,000 ton per yearwith a current density of 4 KA/M2.
Combination plant using the output of the membrane cells as
fed to diaphragm cells might result in considerable cost
reduction.
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SODIUM HYDROXIDE: MEMBRANE
CELLS8/
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AdvantagesMore concentrated brine can be used
Purer and concentrated products (28% NaOH
containing 50ppm of NaCl, 40% NaOH product) are
produced.Saving of energy and transportation cost
Low production cost
DisadvantagesReadily clogged of membrane
Pretreatment of brine is required to remove calcium
and magnesium salts
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Raw materialsBasis: 1 ton of NaOH (76%), 879kg Cl2, 274.7m3H2
Salt 1600kg(sea water, salt lake
and sub soil water)
Sodium carbonate29.2kg(Solvays process, dualprocess or electrolytic process)
Sulfuric acid 100.5kg(Contact process)
Steam 10060kg
Electricity 1197kJ
Refrigeration 910kg
Direct labour 20work-h
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Reaction
NaClNa++ Cl H = + 97.2kcals
2H2O + 2eH2+ OH H = + 68.3kcals
At cathode2H2O + 2eH
++ 2OH
Na++ OH NaOH H = + 112.0kcals
At Anode
CleCl
Cl + ClCl2
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Purification of BrineBrine purification is essential for getting pure caustic soda andto
decrease clogging of the cell diaphragm by insoluble hydroxides formed
during electrolysis.
Brine contains impurities such as calcium, magnesium and iron
compounds.These impurities are removed by adding lime and soda ash, when insoluble
carbonates and hydroxides are precipitated.
Sometimes sulfate are removed with BaCl2or the hot brine is treated with OH
and CO32ions.
After the treatment brine is allow for settling to separate the impurities
and then neutralized with hydrochloric acid.
The saturated brine containing 324gms/liter of NaCl is fed to the cell at600C.
The electrolysis is carried out in diaphragm cells; each cell usually
required 3.0-4.5 volts.
A number of them are put in series to increase the voltage of a given
group.
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Brine electrolysis is carried out with an anode current density of 0.07amp/cm2.
Na+ions formed by electrolysis are moved to the cathode, where H+ions
and OH ions are also formed as a result of reduction of water.
On the other hand Cl ions are directed towards the anode, where they
lose one electron each and form chlorine molecules which liberate aschlorine gas at the anode.
Since the discharge potential of chlorine ions is lower than that of OH
ions, Cl ions are discharge at the anode and OH ions are remain in
solutions.
Similarly the discharge potential of Na+is greater than H+ions hence H+
ions are discharge at the cathode, while Na+ions remain in the solutions.Chlorine attack caustic soda solution, resulting into sodium chloride and
hypochlorite:
2NaOH +Cl2NaCl + NaClO +H2O
To preclude the reaction, it is necessary that NaOH and Cl once formed
do not come in direct contact with one another.
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Evaporation and salt separationAbout 10 to 15% caustic soda solution along with some
unconverted NaCl is obtained after electrolysis.
The decomposition efficiency of the cells being in the
range of only 50%, about half of NaCl remainsunconverted and is recovered by reason of its low
solubility in caustic soda solutions after concentrations.
Hence,the weak caustic soda solution is first
concentrated to 50% in a double or triple effect
evaporator so that NaCl completely separated which is
recycled.
The liquid obtained from the salt separator is 50%
caustic soda solution containing 2% NaCl and 0.1 to
0.5% NaCl on a dry basis.
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Final Evaporation50% NaOH solution is concentrated in huge cast iron pot on open
fire.
Approximately 99% water is removed and molten caustic soda is
formed at 5000C to 6000C.
Now a days these pots are replaced by dowtherm heated evaporators
for caustic evaporation about 50%.
Another method of dehydrating 50% caustic soda is the precipitation
of NaOH.H2O by adding ammonia which also purify the caustic soda.
If 50% caustic soda is treated with anhydrous ammonia in pressure
vessels in a counter current manner, free flowing anhydrous crystals
of NaOH separate out from the resulting aqua ammonia.The hot anhydrous caustic is treated with sulfur to precipitate iron
and then allowed to settle.
Then a centrifugal pump is lowered by crane in the molten NaOH
and the liquid is pumped out in to thin steel drums.
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Purification of caustic soda50% caustic soda solution still contains impurities
such as colloidal iron, NaCl and NaClO.
Iron is removed by treating caustic with 1% by weight
of 300 mesh CaCO3and filtering the resultingmixture through a filter on CaCO3per coat.
Sodium chloride and hypochlorite are removed by
dropping the 50% caustic solution through a column
of 50% NH4OH.
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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Chlorine DryingThe hot chlorine evolved from the anode compartment contains much
water vapour.
Therefore, it is cooled to condense most of the water vapour and further
dried in the sulfuric acid scrubber (stoneware tower or stainless steel
tower with acid proof packing).
The dried CI2is compressed between 35 to 80 psi by one of the followingtemperature pressure combination.High pressure (910 atm), water coolingMedium pressure (23 atm), refrigeration at 200CLow pressure (310 cm Hg ), refrigeration at 400C
Rotary compressors with H2SO4seals have been used for liquefaction
process.The heat of compression is progressively removed by water and finally by
refrigeration to about 290C, when all the chlorine should be liquefied.
It is further cooled 450C and the liquid chlorine is led to a steel storage
tank and then filled in steel cylinder of 50-100 kg capacity for sale.
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SODIUM HYDROXIDE (NAOH): PROCESS
USING DIAPHRAGM CELL8/
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HydrogenHydrogen evolved at the cathode is either burnt for
boiler fuel or used as hydrogen source.
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS8/
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Raw materials Basis: 1 ton Sodium hydroxide
Sodium carbonate 1360kg(Solvays process, dual process or
electrolytic process)
Lime 75kg (from mineral calcite or aragonite,
which can be used after removal of clay,slit and sand (silica).)
Water 1000kg
Steam 1225kg
Fuel 13000000 Btu
Electricity 19KWH
Reaction
Na2CO3+ Ca(OH)2NaOH + CaCO3
SO O (AO ) OC SS
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS8/
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SODIUMHYDROXIDE(NAOH)PROCESS
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS8/
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SODIUMHYDROXIDE(NAOH)PROCESS
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS8/
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Series of causticizer tank or agitator are added with 20% solutionof soda ash (made with weak liquor from a previous stage) and
milk of lime or slaked lime.
The causticizer tanks containing steam line to heat the mixture to
80900C are either fitted with mechanical stirrer or compressed
air as substitute of agitator.After the equilibrium the liquid is allowed to settle for 23 hours .
The clear liquid containing about 10% NaOH is drawn by a swing
pipe.
The sludge is washed in counter current manner with the washing
of the previous operation.
The causticising process is completed in the series of three
agitators.
The mixture of NaOH and CaCO3from the last agitator is charged
to the first Door thickener, which consist of a large shallow
cylindrical tank into which the slurry is fed at the center.
SODIUMHYDROXIDE(NAOH)PROCESS
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS
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The over flow liquid from the first thickener is filtered and filtratecontaining 10-11% NaOH is evaporated to 50 % solution in a triple effect
vacuum evaporator.
The solid CaCO3is gradually settles to the bottom.
The lime sludge from the bottom of the first thickener is washed with the
water.
The filtrate from the next operation is also added to the second thickener,
where the liquor is treated with excess of weak soda solution.
The overflow from the second thickener is used as a weak liquor to make
soda ash solution.
The lye suspension from the thickener is filtered through rotary drum
vacuum filter and passed to a third thickener where it is finally washed
with fresh water to remove any traces of NaOH.
The slug of the filter cake (CaCO3) is return in the lime kiln to from lime.
The caustic soda (11 % strength) contains small amount of NaCl and
Na2CO3.
SODIUMHYDROXIDE(NAOH)PROCESS
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS
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Kinetics and thermodynamicsEffect of temperature
Temperature effects on both equilibrium yield and rate of
reaction.
Concentration of reagents favourable to the process
Na2CO3+ Ca(OH)2NaOH + CaCO3 The equilibrium constant of above reaction is
---- (1) Since calcium carbonate and calcium hydroxide are only slightly soluble, their
solutions are always saturated and concentration of two components in the solution
is therefore constant.
Equation (1) can be written as
---- (2)
[ ][ ][ ] ( )[ ]
232
2
3
OHCaCONa
NaOHCaCOK
c =
[ ][ ]
32
2
CONa
NaOH'kc=
SODIUMHYDROXIDE(NAOH)PROCESS
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS
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The yield of NaOH is given by,
---- (3)
The ratio which appears in the denominator of
(3), when use is made of (2) is equal to the other
ratio [NaOH]/K'c. On the basis of this (3) becomes
---- (4)
[ ][ ] [ ] [ ]
[ ]NaOHCONaCONaNaOH
NaOHNaOH
3232 2
1
2+
=
+
=
[ ] [ ]NaOH"K'k
NaOHNaOH
c
c
+
=
+
=
1
1
21
1
SODIUMHYDROXIDE(NAOH)PROCESS
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SODIUM HYDROXIDE (NAOH): PROCESS
LIME SODA PROCESS
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It is seen from (4) thatthe yield of NaOH ishigh when the concentration of the same
hydroxide at equilibrium is low, i.e. when
starting concentration of sodium carbonate
is small.In practice it is necessary to work with stating
solutions which are not too dilute in order to
avoid excessive cost of concentrating the produced
caustic soda solutions.Generally,solution containing 12-14% of
sodium carbonate are used.
SODIUMHYDROXIDE(NAOH)
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SODIUM HYDROXIDE (NAOH):
PROPERTIES
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Molecular formula : NaOHMolecular weight : 39.997gm/mole
Appearance : White, waxy, opaque crystal
Odour : Odourless
Boiling point : 13880C
Melting point : 3180C (Decompose)
Density : 2.13gm/mL
Solubility : Soluble in waterIt is hygroscopic in nature
SODIUMHYDROXIDE(NAOH)
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SODIUM HYDROXIDE (NAOH):
APPLICATIONS
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It is an important heavy chemical and occupiesamong the basic chemicals position equal in
importance to sulfuric acid and ammonia.
It is used insoap, rayon, dyes, paper, drugs,
foods, rubber, textiles, chemicals, bleaching,metallurgy and petroleum industries.
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CHLORINE (CL)Using diaphragm cellsDeacons method
Other methods
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CHLORINE (CL): INTRODUCTION
Chlorine (Cl) is a chemical element having atomicnumber 17.
It is the second lightest halogen after fluorine.
The element forms diatomic molecules under STP,
called dichlorine.It has the highest electron affinity and the third highest
electro-negativity of all the elements; for this reason,
chlorine is a strong oxidizing agent.
Chlorine can be manufacture by several methods such
as electrolysis, Deacons, heating of auric acid andplatonic chloride.All methods except electrolysis are costly.
Therefore, chlorine is largely manufacture by electrolysis
process
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CHLORINE (CL): INTRODUCTION
In 1630, Jan Baptist van Helmont was the first who obtainedthe Chlorine gas.
The synthesis and characterization of elemental chlorine
occurred in 1774 by Carl Wilhelm Scheele, who called it
"dephlogisticated muriatic acid air," having thought he
synthesized the oxide obtained from the hydrochloric acid.Because acids were thought at the time to necessarily contain
oxygen, a number of chemists, including Claude Berthollet,
suggested that Scheele's dephlogisticated muriatic acid air
must be a combination of oxygen and the yet undiscovered
element, and Scheele named this new element within this oxide
as muriaticum.
In 1809, Joseph Louis Gay-Lussac and Louis-Jacques proved
that this newly discovered gas was the simple element which
was reconfirmed by Sir Humphry Davy in 1810, who named it
chlorine, from the Greek word chlros meaning "green-yellow."
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CHLORINE (CL): PROCESSES
Using diaphragm cellsChlorine can be obtained as co-product during the manufacture of caustic soda
by electrolysis process.
Deacons methodHCl is partially oxidizes to chlorine by heating of HCl gas with oxygen (air) at
4004500C in presence of porous earthenware impregnated CuCl2as catalyst.
4HCl + O22Cl2+ 2H2O
OR
2CuCl22CuCl + 2Cl2
4CuCl + O22Cu2OCl2
Cu2OCl2+ 2HCl2CuCl2+ H2O
Cl2mixed with unconverted HCI and system is washed with cold water and
dried with conc. H2SO4. This is an old method for manufacture of chlorine and
is not in used now.
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CHLORINE (CL): PROCESSES
Pure chlorine can also be prepared by heatingAuric chloride (AuCl3)or platonic chloride
(PtCl4)in a hard glass tube.
175oC 190oC
2AuCl32AuCl + 2Cl2Au + 3Cl2
375oC 600oC
PtCl4PtCl2+ Cl2Pt + 2Cl2
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CHLORINE (CL): PROPERTIES
Molecular formula : Cl2Molecular weight : 70.906 gm/mole
Appearance : Yellow green gas
Odour : Similar to house hold bleach
Boiling point : -40C
Melting point :1010C
Vapour density : 2.48 (v/s air)
Vapour pressure : 4800mmHg (200C)
In upper atmosphere, chlorine containing molecules such as chlorofluoro-carbons
have been implicated in ozone depletion.
Elemental chlorine is extremely dangerous and poisonous for all life forms
It is necessary to most forms of life, including humans, in form of chloride ions.It is the only acidic gas which turns damp blue litmus red and bleaches it to
white.
It is two and a half times heavier than air. It becomes a liquid at 34 C.
The affinity of chlorine for hydrogen is so great that the reaction proceeds with
explosive violence in presence of light
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CHLORINE (CL): APPLICATIONS
Chlorine is usedfor producingsafe drinking water.
for the manufacture of chlorates and it is important in organic
chemistry, forming compounds suchas chloroform, carbon
tetrachloride, polyvinyl chloride, and synthetic rubber.
indyestuffs, petroleum products, medicines, antiseptics,insecticides, foodstuffs, solvents, paints and plastics.
as anoxidizing agentand in substitution reactions.
Inpaper and pulp, solvents, explosives, plastics,
pesticides and sanitation.
Chlorinated compounds are used mostly for sanitation andtextile processing.
As a common disinfectant, chlorine compounds are used in
swimming pools to keep them clean and sanitary.
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ACKNOWLEDGEMENT
Slides are developed from the following references:Austin G. T., "Shreves Chemical Process Industries",
Fifth edition, Tata McGraw Hill, NY.
Kent J.A.,"Riegel's Handbook of Industrial
Chemistry,CBSPublishers.Gopala Rao M. & Marshall Sittig, "Drydens Outlines of
Chemical Technology for the 21stCentury", Affiliated
East West Press, New Delhi.
Mall I. D., "Petrochemical Process Technology",
Macmillan India Ltd., New Delhi.http://nptel.ac.in/courses/103106108/24
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